Cell-cell adhesion defines solid tissues, and dysregulation of adhesion is an essential step in cancer cell metastasis. The protein aE-catenin has critical roles in cell and tissue development by transducing mechanical tension between cadherin cell adhesion molecules and the actin cytoskeleton into biochemical signals. We will investigate the molecular basis of how aE-catenin structure changes in response to force, and how its molecular behavior contributes to the formation and dissociation of cell-cell contacts. Our approach is to use a combination of rigorous biochemical characterization (Weis) and innovative single-molecule optical trapping assays (Dunn) to discover how the protein a-catenin both reinforces cell-cell junctions and triggers downstream signal transduction in response to mechanical stress. This question has deep biomedical significance, since a-catenin is known to be required for the formation of multicellular tissues and is a central player in both organogenesis and cancer metastasis. Cell biological data show that a-catenin and its binding partner -catenin are required to link the intracellular adhesion protein E-cadherin (epithelial cadherin) to the actin cytoskeleton. However, the a- catenin/-catenin/E-cadherin does not bind actin in bulk biochemical assays. In preliminary work, we used a novel single-molecule optical trap assay to show that the cadherin/catenin ternary complex can indeed bind actin, but only in the presence of mechanical load. Further, we find that the strength of the a-catenin-actin bond increases with mechanical load, and that binding of the cadherin/catenin complex to the actin filament is highly cooperative. The implication of these findings is that a-catenin acts as a force sensitive linker that can reinforce cell-cell contacts in response to mechanical load. This mechanism provides an elegant means to maintain tissue integrity in the presence of mechanical strain, and provides an explanation for how cells may sense tension at cell-cell junctions, a topic of intense current interest. However, how exactly a-catenin senses mechanical tension is not known. We will use a combination of biochemical and single-molecule biophysical approaches to: 1) determine the molecular mechanism by which a-catenin forms a force-sensitive linkage between cadherins and the actin cytoskeleton; and 2) discover how cooperative structural transformations in a-catenin, actin, or both regulate binding between the cadherin/catenin complex and filamentous actin. These measurements will reveal the molecular mechanism by which a-catenin senses force at cell-cell junctions. In addition, this work will provide a mechanistic basis for understanding how groups of cadherin-catenin complexes work in concert to remodel cell-cell junctions in response to changes in mechanical load, with potentially broad implications for our understanding of epithelial remodeling and morphogenesis.